1. Field of the Invention
The present invention relates to a solid-state imaging device and a method for producing the same. More particularly, the present invention relates to: a solid-state imaging device in which a pixel-by-pixel array of photoelectric conversion sections generate electrical charges in accordance with an amount of incident light (as in the case of a CCD, or “Charge Coupled Device”), and the electrical charges generated by the photoelectric conversion sections are respectively transferred by charge transfer sections for outputting an electrical signal; and a method for producing such a solid-state imaging device.
2. Description of the Background Art
In recent years, solid-state imaging devices (e.g., CCDs) have seen remarkable improvements in terms of increase in the number of pixels and downsizing of the device. In a solid-state imaging device in general, as compared to the area of each pixel, a corresponding aperture provided in a photoshield film, which is provided above the photodiode sections, is relatively small. Moreover, as the pixel size decreases (for example, to about 3 μm×3 μm or less), the absolute amount of light which is received by each photodiode section becomes more reduced as compared to conventional cases. Therefore, any incoming light for each pixel must be efficiently led through the corresponding aperture.
In order to efficiently collect the incoming light for each pixel onto an aperture, a conventional solid-state imaging device is provided with microlenses (hereinafter referred to as “upperlenses”),which are disposed upon a color filter. Recently, in order to further enhance the focal power and attain improved sensitivity, it is coming into practice to provide microlenses (hereinafter referred to as an “intralayer lens”) not only above but also below the color filter (see for example, Japanese Patent Laid-Open Publication No. 2000-164837 (page 7, FIG. 1)). By employing such two groups of two microlenses, there is also provided an additional advantage of reducing wavelength-dependent displacement of focal points due to chromatic aberration.
FIG. 18 shows an exemplary cross-sectional structure of a conventional solid-state imaging device which includes upper lenses and intralayer lenses. In FIG. 18, a photoshield metal film 110 prevents gate electrodes 108 and charge transfer sections 106 from being irradiated with light. In order to allow light to impinge on photodiode sections 104, the photoshield metal film 110 has apertures formed above the respective photodiode sections 104. Each upper lens 122 and intralayer lens 530 converge light onto a corresponding photodiode section 104. A color filter 120 is provided between the layer of upper lenses 122 and the layer of intralayer lenses 530.
A distance 552 from the surface of each photodiode section 104 to each upper lens 122 is preferably short. As the distance 552 becomes longer, problems will emerge such as light which has been led through the upper lens 122 and the intralayer lens 530 being intercepted by the photoshield metal film 110, or light leaking into adjoining pixels. For example, in the solid-state imaging device shown in FIG. 18, among light rays 562, 564, and 566 which have traveled through an upper lens 530, the ray 566 is intercepted by the photoshield metal film 110. If this happens, the amount of light which is received by the photodiode section 104 becomes smaller than the amount of light entering through the upper lens 122. If the ray 566 which has been intercepted by the photoshield metal film 110 somehow (directly or by reflection) strays into another pixel, the problem of intermixing of colors will occur.
Reducing the thickness of the color filter 120 to decrease the distance 552 from the surface of the photodiode section 104 to the upper lens 122 is not preferable because it will result in a degradation of the spectrometric characteristics. The color filter 120 needs to have a certain thickness or more in order to attain predetermined spectrometric characteristics, and thus, the distance between the upper lens 122 and intralayer lens 530 cannot be reduced beyond the constraints imposed by the thickness requirement for the color filter 120. This in turn hinders reduction in the distance 552 from the surface of the photodiode section 104 to the upper lens 122. Furthermore, if the distance between the upper lens 122 and the intralayer lens 530 is increased due to the thickness requirement of the color filter, the curvature of the intralayer lens 530 must be made greater than the curvature of the upper lens 122. If, conversely, the curvature of the upper lens 122 is greater than the curvature of the intralayer lens 530, a diffused component of the light which has been excessively throttled or focused by the upper lens 122 will strike the intralayer lens 533, thus making it difficult to create a convergence spot on the surface of the photodiode section 104. In order to increase the curvature of the intralayer lens 530, it is necessary to increase the thickness of the intra layer lens 530. Increasing the thickness of the intralayer lens 530, however, leads to a further elongation of the distance 552 from the surface of the photodiode section 104 to the upper lens 122.